Two-dimensional (2D) dimming backlight technology is used for liquid crystal display television (LCD-TV) applications to, for example, improve the contrast and black levels of the display panel, as well as to reduce power consumption. The individual light-emitting diodes (LEDs) that make up an LED array in an LCD display may possess a wide spread of physical characteristics between the individual LEDs, due to, for example, variances in manufacturing within the LED array. The varying physical characteristics between individual LEDs may include forward voltage (Vf), luminance (i.e., brightness), power efficiency, and dominant wavelength. LED characteristics, such as color (i.e., white backlights vs. RGB colored lights) and luminance, may be adjusted relative to other individual LEDs in the LED array, or may be adjusted to adhere to a product specification.
During regular operation, the performance and operational lifetime of an individual LED may degrade when its junction temperature becomes overheated. An LED might become overheated due to the amount of power driven to it to, for example, increase the LED's luminance. Because luminance is directly proportional to power (and therefore, resistance) and temperature, a higher luminance output may cause the LED to overheat and degrade over time. As a result, many LED arrays are designed to avoid degradation, wherein individual LEDs are driven to only produce a nominal luminance, which may be defined as 100% luminance at maximum allowable temperature, during regular operation. Because of varying physical characteristics, an LED array may be designed for the worst-case scenario, guaranteeing the nominal luminance of the weakest LED, which may be defined as the individual LED in an LED array that has the smallest maximum temperature. A temperature sensor can be added to the system, which measures the temperature close to the LED and provides a signal to a feedback loop to adjust the current in order to stabilize the flux output as well as to prevent excessive LED temperatures. This allows for a higher luminance output and makes the system more efficient. The LED junction temperature can be measured more efficiently when using a forward voltage measurement and the known relationship between the forward voltage and temperature.
The control system of an LCD television may therefore drive an LED array towards a uniform luminance and color, while limiting the maximum luminance output of the entire LED array to only the nominal luminance of the weakest LED. During normal operation, a control circuit may manage the luminance of an LED array through the use of a pulse-width modulated (PWM) signal to control current delivered to the LEDs of the array. The PWM signal delivered by the control circuit may have a duty cycle with a range from 0-100% and may be directly proportional to the luminance, with a 100% duty cycle corresponding to a 100% luminance. A control circuit may then adjust the duty cycle of the PWM to limit the power delivered to the LED array.
While the control system for the LED array may guarantee operation in the worst-case scenario by guaranteeing against degradation for the weakest LED in the array, this design principle may also unnecessarily limit the possible luminance of other LEDs in the array, most of which are capable of outputting light at much higher luminance levels due to a higher maximum allowed temperature. Furthermore, the PWM control that regularly dims the array also dims the luminance of the more capable LEDs to much lower than their capacity. This may be an inefficient use of resources, as the arrangement of an LED array may severely limit a large number of more capable LEDs due to a limiting smallest maximum temperature. For example, the uniform design may limit both the brightness (maximum luminance) and contrast (range of luminance) when consuming a given amount of power. In view of the foregoing, it would be desirable to drive individual LEDs in an LED array beyond nominal luminance of the weakest LED.